System
Capacity or Volume
Can small cabs move large numbers of
people like traditional mass transit? Yes. Uninterrupted flow is the
key to capacity, not vehicle size. For example, 60passenger buses
arriving two minutes apart (a very high flow rate for an American bus
system) can carry 1800 passengers per hour. PRT vehicles coming every
two seconds can provide the same capacity. PRT capactiy depends
on headways:

0.5 second = 120/min or 72000hr
 0.6 second = 100/min or 6000/hr
 2.0 seconds = 30/min or 1800/hr
A commonly accepted safety zone on
roadways is 2 seconds between cars. Although automatic control of PRT
cabs is safer and more reliable than human drivers, let's assume our
PRT systems starts with that comfortable two seconds of space between
each cab, aka "headway". At that headway, 1800 cabs per
hour can roll down the guideway. That's 1800 people per hour assuming
soleridership will prevail (30 cabs/min * 60 mins/hour = 1800 cabs
per hour). That approximates the maximum volume of a freeway lane of
traffic (2200). After a few years of operation, we may have the
confidence to reduce the headway times to only one half second. That
would quadruple throughput to 7200 cabs/hour. Now we're talking the
volume of three freeway lanes in less than the space of one physical lane.
Now, compare that volume to LRT and
trains. Although LRT systems may be designed for high volume, the
actual limit of any operating LRT system in the U.S. is 1200 riders
per hour; peak in Sacramento is about 1000 passengers/hr.
Likewise for trains where the theoretical limit is 20,000
riders/hour, actual loading often tops out near 7000 riders/hour. An
exception may be BART where reports indicate nearsaturation of the
transBay tube at 20,000 riders/hour [is that one way, or both?].
Another capacity comparison could be made with computer controlled
cars as demonstrated near San Bernadino, CA. Partners for
Advanced Transit and Highways (PATH) ran Buick Le Sabres by computers
on a dedicated strip of freeway with magnets embedded so the cars
could be computer controlled. They ran for thousands of miles at 60
mph with 0.25 sec. headways. Some of PATH's research,
particularly its work in the Advanced Vehicle Control Systems area,
has been covered by a range of media. http://www.path.berkeley.edu/PATH/Publications/Media/
More recently,
Hundai has demonstrated even more control of vehicles at speed.
Speed is another factor in capacity. Here
are critical ideas from PRT pioneer Ed Anderson:
Subj: RE: [prttalk] Digest Number 56
Date: 5/27/01 5:32:19 PM Pacific Daylight Time
From: jeanderson@taxi2000.com (Ed Anderson)
You mentioned some of the system problems.
Tires vs. maglev are not the most important considerations. Curve
radii increase as the square of the speed and offline guideway
lengths increase in proportion to speed. These are the most important
factors. Lifecyclecost per passengermile is the annualized capital
+ operating cost divided by the annual ridership. Costs increase with
speed regardless of the means of suspension and ridership will
increase with speed to a point. After a certain speed, costs increase
faster than ridership so the cost per passengermile increases.  JEA
So, pick a speed that ensures high
ridership by offering 1) a low cost per passengermile and 2) speeds
that compete with the automobile . Absent any analysis, I pick 40
mph. Let's start engineering with that operating speed in mind.
Here's some capacity numbers from the bike
folks: It takes three lanes of a given size to move 40,000 people
across a bridge in one hour using automated trains, four to
move them on buses, twelve to move them in their cars,
and only two lanes for them to pedal across on bicycles.
A vehicle at a red light requires about
240 square feet of space (that's a standard 12foot lane with a
standard 20foot long "envelope" per car). At 20 mph, it
requires about 700 square feet. And for a car zooming at 40
mph, the number balloons to about 2,000 square feet. Maximal
traffic on a highway lane runs at 2.2second intervals. At 10
mph that is 1000/hr; at 30 mph about 1500/hr. It never gets
more about 1500/hr because the vehicle grows with velocity. At
50 mph, a car is 1,285 feet long. PRT capacity or speed
does not decrease with a heavy load; at 2 second headway, it will
have 3 times the capacity as a landeof traffic, and at 0.5 second
headway, it will have 12 times the capacity.
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